1. Field of the Invention
The present invention generally relates to optical devices, and more specifically, to an optical device used in technical fields such as optical communication or optical signal processing.
2. Description of the Related Art
As for recent and continuing development of optical communication systems, a system having a large capacity and high functions has been requested. Hence, it is required to develop an optical device such as an optical switch or modulator for high speed optical wave. In such an optical device, not only high speed but also low loss or low voltage is required.
As the high speed optical device, for example, an optical switch, an optical modulator, and an optical control element are reported wherein a waveguide is formed in a substrate made of a crystal of lithium niobate (LiNbO3) having a large electrooptic coefficient and the change of the index of refraction of the waveguide is controlled by changing it in an electric field by using the electrooptic effect.
The optical waveguide having a structure where a metal such as titanium (Ti) or the like is diffused in the crystal of lithium niobate (LiNbO3) obtains low propagation loss equal to or less than 0.1 dB/cm for a wavelength of 1550 nm.
However, in order to use the optical device using such a diffusion waveguide (optical waveguide) in an optical fiber transmission system, it is necessary to consider the coupling loss with the optical fiber.
In addition, in a case where the optical waveguide is three-dimensionally formed on the dielectric substrate by using a femto-second laser or high intensity laser, it is well known that the cross section of the waveguide becomes elliptic. Because of this, it is necessary to consider the coupling loss similar to the above-mentioned diffusion waveguide.
FIG. 1 is a schematic view showing a light intensity distribution of light propagating in an optical fiber and a light intensity distribution of light propagating in a diffusion waveguide of an optical waveguide element, in the related art.
The intensity (mode) distribution of the light propagating in the optical fiber is circular shaped as shown in FIG. 1-(a). On the other hand, in the optical device using the diffusion waveguide, as shown in FIG. 1-(b), the distribution of the indices of refraction between the direction perpendicular to the substrate, namely substrate depth direction, and the direction parallel to the substrate are different. The intensity distribution of the light is extremely different from the circular shape and is substantially elliptical in shape.
FIG. 2 provides graphs of the distribution of the change of the index of refraction in the direction perpendicular to the substrate, namely the substrate depth direction, and parallel to the horizontal with the substrate, in the related art optical device.
The intensity distribution of the light in the diffusion waveguide is defined by a principle of the diffusion. The diffusion waveguide receives diffusing atoms from an upper side. Hence, the distribution of the index of refraction horizontal (parallel) to the substrate has, as shown in FIG. 2-(b), a substantially symmetric shape. On the other hand, the distribution of the change of the index of refraction in the direction perpendicular to the substrate, namely the substrate depth direction is, as shown in FIG. 2-(a), leans (is skewed) toward the surface of the substrate. Because of this, when the diffusion waveguide is inserted between the optical fibers, coupling loss of approximately 2 dB in total at the input and output parts may be incurred.
Therefore, in order to reduce the coupling loss between the diffusion waveguide and the optical fibers, it is necessary to make the distribution of the change of the index of refraction of the diffusion waveguide at the optical input and output parts similar to the substantially circular-shaped light intensity distribution of the optical fiber.
On the other hand, in the optical control part, it is possible to obtain high electric field application efficiency when the light intensity distribution leans toward the vicinity of an electrode having a high strength applied electronic field.
In order to solve the above-discussed problems a method is suggested in Japanese Laid Open Patent Application Publication No. 62-103604 whereby a titanium (Ti) diffusion waveguide is formed and then magnesium oxide (MgO) is additionally diffused so that the index of refraction of the surface is decreased.
In addition, Japanese Laid Open Patent Application Publication No. 2005-284256 discloses a waveguide-type optical splitter having a structure where a waveguide for input, plural waveguides for output and a slab waveguide are formed on a substrate. The slab waveguide has an incident end and an output end. The output end has a circular shape centered around the incident end or its vicinity. The waveguide for input is connected to the incident end and the plural waveguides for output are connected to the output end. The waveguide for input is connected to the incident end via a waveguide, whose opening width is narrowed and tapered.
Furthermore, Japanese Patent No. 2793562 discloses a structure where an optical waveguide is formed on a substrate of lithium niobate (LiNbO3) and the optical waveguide is formed by effecting thermal diffusion of titanium (Ti).
In addition, Japanese Patent No. 2817769 discloses a semiconductor optical amplifier device including a semiconductor laser section and a semiconductor optical amplifier both of which are formed on the same semiconductor substrate and which are coupled to each other, the semiconductor optical amplifier including a waveguide layer formed on the semiconductor substrate and a tapered electrode formed on an upper surface thereof, the semiconductor optical amplifier being supplied with an incident laser beam from the semiconductor laser section through an incident surface and amplifying the incident laser beam to emit an amplified laser beam as an output laser beam through an emission surface, the tapered electrode spreading toward the emission surface.
However, in the process discussed in Japanese Laid Open Patent Application Publication No. 62-103604, after titanium (Ti) is diffused, deposition and thermal diffusion of magnesium oxide (MgO) are required and therefore the number of the processes is increased.
Furthermore, in a case where lithium niobate (LiNbO3) is used as a dielectric substrate, out diffusion of lithia (Li2O) from inside of lithium niobate (LiNbO3) is expected due to the diffusion of magnesium oxide (MgO). Therefore, it is difficult to achieve a desirable refractive index profile.
In addition, for example, if the out diffusion of lithia (Li2O) happens too much, the ratio of lithium (Li) and niobium (Nb) in a crystal may be changed and therefore a decrease of electrooptic coefficient would be expected.